1. Field of the Invention
The present invention relates to making optical articles such as opthalmic lenses or substrates for compact disks by molding and, in particular, to using a continuous extrusion-compression molding method to make plastic lenses whereby a polymer melt is fed from an extruder or melting apparatus to a first die of a plurality of rotating sequentially processed compression die sets comprising a first die and a second die positioned on a rotating turntable or conveying system, the lenses formed by compressing the die sets, the lenses separated from the dies and the dies recycled to die loading and the melt feed step of the process and the process repeated until the desired number of lenses are produced.
2. Description of Related Art
Direct compression molding of thermoplastic and thermoset polymers to make plastic optical articles such as lenses has been used for years. Basically, the compression molding process uses a packed powder or a pre-form material and the article is formed by adding the material to a mold, closing the mold and pressing the mold at an elevated temperature. The mold typically consists of a lower die and an upper die forming a mold or die set. In a thermoplastic lens compression molding process, the material and the mold are heated to a predetermined temperature to soften the material and a compressive force applied to the mold for a predetermined time to reach a predetermined cavity size with excess material being squeezed out of the mold. The mold is then cooled and opened and the article removed from the mold. This method, however, is not commercially attractive because it has a long cycle time, poor energy efficiency and it is economically less beneficial compared with an injection molding process. A direct injection molding process, in general, is a faster and more efficient method for molding thermoplastic materials.
In molding such optical articles as a negative powered Rx lense, however, conventional injection molding methods have a number of serious operational problems. These type lenses have a cross section in which the center is thinner than the edge and this feature makes direct injection molding very difficult especially when the viscosity of the melt is high. In general, the melt injected into the mold tends to fill the edge of the mold first due to less resistance in the thicker section of the mold cavity and after filling much of the edge area, the melt redirects itself to the center thinner area of the mold cavity. The mold used in an injection molding process has to be relatively cold due to the need for shortening the cycle time and when the melt meets in the center area a knit line is often formed. This phenomenon is especially serious when molding a lens with thinner center thickness which has higher value due to its lighter weight. In addition, injection molding typically introduces orientation-induced stress and birefringence. Lack of sufficient packing force used in cold runner injection molding technology often results in uneven shrinkage related lens pre-release and causes line marks on the lens surface. These type problems have to be avoided for the article to be suitable for optical uses.
Other type optical articles which can be made using the method and apparatus of the invention include plastic substrates used to make compact disks used in CD-ROMS, CD players, etc. In general, the substrate is a circular planar disk up to about 1.8 mm thick and is made from a plastic such as polycarbonate. As is well known in the art, the plastic disk substrate is plated or coated with recording layers. Conventional injection molding has been found to be very difficult and injection-compression molding is typically used.
To overcome some of the above problems, injection-compression molding technology has been developed as shown in U.S. Pat. Nos. 4,008,031; 4,091,057; 4,254,065; 4,364,878; 4,409,169; 4,442,061; 4,519,763; 4,540,534; 4,627,809; 4,707,321; 4,828,769; and European Patent Publication 0130769, which patents are hereby incorporated by reference. Generally, a shot of melt is injected into a separated mold using an injection molding machine. The mold is then closed to apply a compressive force to the melt and packs the mold cavity by hydraulic clamping or from an auxiliary component such as springs. The timing for the injection and compression steps, melt shot size and temperature control of the mold are all critical operational factors. In many cases, hot runner technology has to be used to gain sufficient packing force.
Even though there are different versions of the injection-compression molding technology, they are more or less similar to each other and this technology has certain drawbacks. For one, the process often produces a product having some stress and birefringence possibly due to the large temperature difference between the mold and the melt. Additionally, the cycle time is often very long because filling of the mold is a slow process and in some improved methods, the mold has to be heated and cooled in every cycle. The use of hot runner technology increases the time the material is exposed to high temperatures and a heat sensitive material may be adversely affected. The cost of the mold is also extremely high and the controlling system to coordinate the injection and compression steps is critical and often very complicated and expensive. Further, in order to make different power lenses, the mold inserts must be changed which causes down time and process inefficiency.
Many polymeric materials are compounded in an extruder by adding to the extruder the polymer in the form of pellets and additives. Some specialized optical materials such as thiourethane and urethane polymers and/or copolymers are made in reaction extruders by adding the polymer reactants to the extruder. When molding an article, the pellets may also be heated in a molding machine and fused into a melt or softened to facilitate the molding process. Typically, however, plastic materials are prone to decomposition at high temperatures and/or in a long heating process. There is also generally a direct relationship between the optical and mechanical properties of polymer lenses and how many times and how long the polymer has been fused, especially for heat sensitive materials. Normally, materials formed into optical articles need to have a thermal history as short as possible, otherwise, increased yellowing, oxidation of the polymer and deteriorated mechanical properties may result. Also from an energy efficiency point of view, re-heating plastic pellets to a molten form is not economically nor environmentally attractive. One of the difficulties of utilizing the melt from an extruder directly as a feeding source to the molding machine is that extrusion is typically a continuous process and the conventional injection molding, injection-compression molding, and compression molding methods are all batch or intermittent processes. To couple a continuous extruder with an intermittent or batch molding machine presents a formidable burden.
In a related application, U.S. Ser. No. 09/014,811 entitled "Extrusion-Compression Molding of Optical Articles" filed Jan. 28, 1998 and assigned to the assignee of the present invention, an extrusion-compression molding method is disclosed which overcomes many of the problems of the prior art.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a continuous method of molding polymers, particularly thermoplastic materials, into optical articles such as Rx lenses and compact disk substrates using an extruder or other apparatus to form a polymer melt and to compression mold the melt to form optical articles without commercially unacceptable defects such as a knit line and pre-release marks and with minimum stress and birefringence.
It is another object of the present invention to provide a continuous method which is energy efficient and has reduced thermal processing time whereby an extruded melt is used directly from a reaction extrusion or other extrusion process to mold an optical article without the additional steps of making pellets and then fusing the pellets to mold the article.
It is another object of the invention to provide a thermoplastic lens molding method in which there is no down time when changing from making a lens of a certain power to another different power.
In another object of the invention, a continuous method is provided for the use of pelletized thermoplastic materials to make optical articles.
It is a further object of the present invention to provide a continuous method whereby an extruder or a reaction extruder and compression molding of a die set comprising at least two die sections are used in sequence to make optical articles such as lenses.
Another object of the invention is to provide an apparatus for continuously making optical articles such as lenses and substrates for compact disks by a molding process.
In another object of the invention, optical articles continuously made using the method and apparatus of the invention are also provided.
An additional object of the invention is to provide a method for post-treating compression molded die sets containing lenses to form optical articles with minimum stress and birefringence which is significantly less than present lenses in the marketplace.
Still other objects and advantages of the invention will be obvious in part from the specification.